In order to record sound signals from two channels, as for the purpose of stereophonic reproduction, the stylus is driven at an inclination of 45.degree. to the disk surface to engrave respective undulations into the two flanks of a V-groove with a 90.degree. vertex angle. The stylus follows a generally spiral path with closely adjoining turns of a given nominal pitch, yet the excursions of the groove flanks due to the modulating signals require some modification of that pitch if the mean spacing of adjacent grooves is to be maintained as small as possible. In general, a predetermined minimum value for the spacing and the depth of the grooves is to be observed.
It is known to generate so-called preview signals which precede the actual modulation signals, picked up from respective tracks of a storage medium such as a magnetic tape, by a certain lead time (preferably on the order of half a disk revolution) designed to allow for a corrective positioning of the stylus support on the basis of the width of the groove previously formed and of the width changes of the current groove due to the forthcoming modulation signals. Thus, the nominal spacing must be increased when the depth (and therefore the width) of either or both of these grooves is larger than normal but can be reduced when it is smaller than normal, thereby allowing better utilization of the available disk area. For this purpose the stylus support is displaceable both vertically--i.e. perpendicularly to the disk surface--and laterally--i.e. horizontally across that surface--under the control of two deflecting signals derived from the aforementioned preview signals. That deflection, however, ought to be so gradual as to avoid audibly perceptible interferences with the recorded sound; thus, the effect of the preview signals upon the deflections of the support is to be spread over the above-described lead time by a conversion of these preview signals into slower-rising deflection signals.
Since the vertical displacement of the stylus support affects both the depth and the width of the groove being cut, it is necessary to take that displacement into account when determining the proper lateral shift of the support. According to the known procedure, therefore, a first and a second signal component derived from the two preview signals are converted--as by integration with suitable time constants--into respective deflection signals of the desired rise time, namely a vertical-deflection signal determining groove depth and a lateral-deflection signal determining groove spacing; the first signal component is also converted into a supplemental signal of the same rise time contributing to the lateral-deflection signal which further contains a basic incremental signal representing the nominal pitch of the spiral path. The incremental signal is summed with the second signal component prior to integration to form a composite signal to which, after conversion into a control signal for the lateral deflector, the supplemental signal is added in part directly (i.e. in real time) and in part after a delay equal to one disk revolution so that the horizontal motion of the stylus support also depends on the width of the groove previously cut.